Boost PFC converters usually adopt constant on time control mode. Under this control mode, the switch is turned on every time when the inductor current crosses zero and is kept on for a fixed time in each switching cycle. That is, the boost PFC converter works at critical mode to reduce switching loss. However, the boost PFC converter has a very high peak switching frequency when the input line voltage crosses zero under this control mode, as shown in FIG. 1. Especially under high line input voltage and light load condition, the efficiency worsens as the switching frequency goes higher.